Due to limited solubility of oxygen in water, it was hitherto necessary to dilute the wastewater with specially prepared aerated dilution water and then carry out the standard BOD test in glass bottles referred to as BOD bottles. One distinct disadvantage associated with this standard BOD test procedure is that the results are not standardised, as the oxygen consumed is dependent on the extent of dilution of wastewater sample.
In the standard test procedure, wastewater, having 10 mg/l ultimate BOD, can be tested. In order to obviate such disadvantage and to determine the BOD of less dilute samples, various constructions of manometric and respirometric instrumental techniques are known in the art. In these methods, dissolution of oxygen, in the respiring liquid, is effected by maintaining gas (Oxygen) phase in contact with the liquid phase. In order to determine the oxygen demand, evolved carbon dioxide, due to biochemical activity, has to be absorbed from the gas phase and the change in pressure or volume provides an estimate of the oxygen consumed.
The disadvantage of such construction is that the instruments are elaborate and, further, means must be provided for causing an agitation of the complete instrument.
Since the apparatus is partially filled and closed, another disadvantage is that of leakages from the reaction vessel.
A further disadvantage is that such a respirometer is not suitable for routine work.
Yet another disadvantage is that evolved carbon dioxide has to be absorbed thereby raising the pH of the reaction mixture thus providing unnatural environmental conditions to the biomass in the respiring liquid.